Myth Busting - Engine Inop Performance

[MauleSkinner]

Finally an explination that makes sense. Thank you! I just had to "rote learn" it. Now I understand.
-mini

I agree...although I understood it (and taught it), Avbug made it elegantly simple.

Fly safe!

David
P.S. His explanation also indirectly points out that the inclinometer is not a yaw instrument, busting yet another myth!

 

[100LL... Again!]

It is doubtful that more than five degrees would be required for a typical GA aircraft for best single engine climb. More could be required under certain circumstances for controllability, but that is another facet to the issue.

Possibly some specialty aircraft or a particularly old design could require an unusually large amount of bank for best OEI rate.

That said, once controllability is satisfied, the bank will probably be closer to 3 degrees than five for best rate.

 

[avbug]

The certification standard and practical test standard is no more than five degrees, within five degrees, and as required.

14 CFR 23 stipulates:

§ 23.147 Directional and lateral control.
(a) For each multiengine airplane, it must be possible, while holding the wings level within five degrees, to make sudden changes in heading safely in both directions. This ability must be shown at 1.4 VS1 with heading changes up to 15 degrees, except that the heading change at which the rudder force corresponds to the limits specified in § 23.143 need not be exceeded, with the -
(1) Critical engine inoperative and its propeller in the minimum drag position;
(2) Remaining engines at maximum continuous power;
(3) Landing gear -
(i) Retracted; and
(ii) Extended; and
(4) Flaps retracted.

§ 23.161 Trim.
...
(d) In addition, each multiple airplane must maintain longitudinal and directional trim, and the lateral control force must not exceed 5 pounds at the speed used in complying with § 23.67(a), (b)(2), or (c)(3), as appropriate, with -
(1) The critical engine inoperative, and if applicable, its propeller in the minimum drag position;
(2) The remaining engines at maximum continuous power;
(3) The landing gear retracted;
(4) Wing flaps retracted; and
(5) An angle of bank of not more than five degrees.

But note the same standard for transport category aircraft certificated under 14 CFR Part 25:

§ 25.161 Trim.
...
(d) Longitudinal, directional, and lateral trim. The airplane must maintain longitudinal, directional, and lateral trim (and for the lateral trim, the angle of bank may not exceed five degrees) at 1.3 VSR1 during climbing flight with -
(1) The critical engine inoperative;
(2) The remaining engines at maximum continuous power; and
(3) The landing gear and flaps retracted.

FAA-S-8081-14A, Practical Test Standards, Private Pilot, Airplane:

A. TASK: MANEUVERING WITH ONE ENGINE INOPERATIVE (AMEL and AMES)
REFERENCES: FAA-H-8083-3; POH/AFM.
Objective. To determine that the applicant:
4. Establishes and maintains a bank toward the operating engine as required for best performance in straight and level flight.

 

[skiddriver]

The 5-degree limit is to keep the manufacturers from generating an artificially low Vmc number during certification testing (see below).

The drag found in the case where the aircraft is kept wings level is due to the sideforce component from the vertical stab after the rotation has stopped. Once you stop the rotation caused by the failed engine by using rudder (with wings level), the vertical stab is generating a sideforce in the direction of the dead engine that has to be neutralized to maintain straight flight. If left unchecked, it will cause you to follow a centripetal path. Additional rudder toward the live engine is required to counteract this, but this induces a sideslip that exposes the dead engine side of the fuselage to the windstream. This causes a righting moment that tends to once again bring the nose of the aircraft towards the dead engine. With sufficient airspeed across the tail plane and enough rudder, these forces can be balanced, but with significant drag caused by the increased induced drag at the vertical stab, and the increased parasitic drag caused by the dead engine side of the fuselage hanging out in the windstream. Your ball will be centered however.

The most efficient mode of flight is to counteract the rotation caused by the asymmetric engine thrust with the rudder, and then use the very small angle of bank required to counteract the residual side force from the vertical stabilizer (normally in the 3-degree range). You can’t see this condition in the cockpit unless you have a direct reading sideslip indicator. If you did, you would center the sideslip indicator using rudder, and then stop residual rotation from tail plane sideforce using angle of bank. Without the sideslip indicator, you set a small angle of bank and neutralize rotation with the rudder. It’s an approximation, but a close one.

You could continue adding angle of bank to a point where you wash out any rudder application and are counteracting the failed engine caused rotation purely with angle of bank. This generates a very large sideslip, is uncomfortable as all get out, increases stall speed and induced drag simultaneously, and can lead to tail plane stall. It does however have the effect of reducing Vmc to a minimum, up until the point where you hit a stall condition (wing or tail plane) and have the aircraft violently depart controlled flight. That’s why the FAA imposed the 5-degree limit.

And finally, as someone who sat through numerous murder boards for real experimental test flight and has been a working experimental test pilot, I encourage everyone to NOT conduct your own test flying out near (or past) the operating envelope of your aircraft. Spending some time trying to figure out whether 2 ½ or 3 degrees of AOB into the live engine is optimal is one thing. But getting outside of the box is just silly and is only going to end up badly.

 

[skiddriver]

It’s interesting to note that the military trained multiengine pilots never hear anything about this GA performance enhancing technique.

Just the ones who didn't pay attention at flight school, it was in the syllabus at the VT's in Corpus.

 

[UndauntedFlyer]

My position on this is that zero-side-slip is an over rated concept that was sold the FAA by some private pilot with a PhD who was trying to make a name for himself.

I have only seen posts by those who report performance enhancements through placebo effect. Or that say they learned of this in school.

Where does the rubber meet the road?

Do your own tests, for real with one engine feathered. Be patient so as to eliminate the placebo effect. See for yourself. With a yaw string you'll see no change either, ball in center or half way out with opposite bank.

If you're patient and ready for the truth it will be shown un to you.

Large multi engine airliners are performance critical at max GW with an engine failure at V1. Is there a word in the FM about zero side slip for the 777 or the 757 or the 767? The answer is NO. Why? Because it's mostly a myth with a sliver of truth. It’s just not worth mentioning

Don't just believe what is said one way or the other. Prove it to yourself as I have done a hundred or more times.

 

[A Squared]

I have only seen posts by those who report performance enhancements through placebo effect. Or that say they learned of this in school.

Interesting indeed that you casually dismiss everyone who has posted information counter to yours as being misled by "placebo effect" I understand what you are alluding to, that people tend to see what the expect to see or want to see. However,

I have two questions:

How can you be so certain you yourself are not being misled by what you refer to as "placebo effect"?

Are you aware that one of the posters whom you so casually dismiss experienced his "placebo effect" while flying instrumented test aircraft in a Navy Test pilot program?

Personally, I'd rate the Navy test pilot in an instrumented plane as a little more "placebo" resistent than yourself, who appears to have an agenda.

 

[UndauntedFlyer]

The following says basic ground school. I agree with everything here.

The drag found in the case where the aircraft is kept wings level is due to the sideforce component from the vertical stab after the rotation has stopped. Once you stop the rotation caused by the failed engine by using rudder (with wings level), the vertical stab is generating a sideforce in the direction of the dead engine that has to be neutralized to maintain straight flight. If left unchecked, it will cause you to follow a centripetal path. Additional rudder toward the live engine is required to counteract this, but this induces a sideslip that exposes the dead engine side of the fuselage to the windstream. This causes a righting moment that tends to once again bring the nose of the aircraft towards the dead engine. With sufficient airspeed across the tail plane and enough rudder, these forces can be balanced, but with significant drag caused by the increased induced drag at the vertical stab, and the increased parasitic drag caused by the dead engine side of the fuselage hanging out in the windstream. Your ball will be centered however.

The following says it is very slight but says nothing about real performance enhancement. I agree it is slight. Not enought to worry about.

The most efficient mode of flight is to counteract the rotation caused by the asymmetric engine thrust with the rudder, and then use the very small angle of bank required to counteract the residual side force from the vertical stabilizer (normally in the 3-degree range). You can’t see this condition in the cockpit unless you have a direct reading sideslip indicator. If you did, you would center the sideslip indicator using rudder, and then stop residual rotation from tail plane sideforce using angle of bank. Without the sideslip indicator, you set a small angle of bank and neutralize rotation with the rudder. It’s an approximation, but a close one.

The following says that if you you use no rudder and all bank that this can be easily seen and felt. I agree with this too and no one needs scientific instruments for this. Any yaw string will show this at that point.

You could continue adding angle of bank to a point where you wash out any rudder application and are counteracting the failed engine caused rotation purely with angle of bank. This generates a very large sideslip, is uncomfortable as all get out, increases stall speed and induced drag simultaneously, and can lead to tail plane stall. It does however have the effect of reducing Vmc to a minimum, up until the point where you hit a stall condition (wing or tail plane) and have the aircraft violently depart controlled flight. That’s why the FAA imposed the 5-degree limit.

The following says that to do the above example could be dangerous. True in some advanced airplanes. Then the following recommends experimenting with the exact demonstration I recommend. Follow this pilots advice. He is does know what he is saying.

And finally, as someone who sat through numerous murder boards for real experimental test flight and has been a working experimental test pilot, I encourage everyone to NOT conduct your own test flying out near (or past) the operating envelope of your aircraft. Spending some time trying to figure out whether 2 ½ or 3 degrees of AOB into the live engine is optimal is one thing. But getting outside of the box is just silly and is only going to end up badly.

In conclusion, everyone says the same thing: It does makes a difference. My point is that it makes no more difference then the rate of climb difference between flying any airplane with normally operating engines with the ball centered or off just a half a ball width. Its just so little difference that its not enough to worry about and can not be see on the VSI or a yaw string. And all this is only when at max power. Of course the slight side slip could be seen on a highly calibrated scientific instrument.

 

[avbug]

Anybody who knows what they're talking about certainly didn't say it makes no difference. It does.

My position on this is that zero-side-slip is an over rated concept that was sold the FAA by some private pilot with a PhD who was trying to make a name for himself.

Funny. It's in my pre wwii manuals, including a wwii aircraft I flew, and not only was it part of our training syllabus...but in every real-life engine failure I experienced in that airplane, it worked, and made a substantial difference.

Gee...were it not for that placebo effect, I'd probably be dead.

 

[UndauntedFlyer]

Placebo effect is powerful.


Avbug: Thanks for your service in WWII.

 

[MauleSkinner]

Its just so little difference that its not enough to worry about and can not be see on the VSI or a yaw string.

Climb rate aside, if you're saying that you can't see a noticable change in a yaw string with half-ball change in slip, you're doing something wrong.

David

 

[UndauntedFlyer]

Maulskinner: Thanks for your interest in this thread. Regarding the yaw string, I went out one time years ago with a camera to try to make a slide to use in my FIRC class on this subject. I was trying to get a photo to look something like the ones in the text books on this subject. But it just didn't work. There was so little change that in order to get something to use I had to fake it with a BIG cross-controlled forward slip. Of course to use that slide would have been a lie so I stopped perpetuating the Myth about all of this great enhancement in performance with the ball half-out vrs. in the center. It is truly overrated, not that the concept isn't true, but just that this whole business is greatly over done. And in the end, a lot of people believe it because of what they have read and then they teach this as something that is important. But really, if a person truly goes out and feathers an engine and then holds Vyse in smooth air you'll see that any performance enhancement is very minimum if can really be detected at all. It is so slight that if you want to see an increase in performance you probably will, and if you want to see no change you probably will see that too. So in the end, the answer is negligible change. This is my experience on this subject.

 

[NoPax]

Zero-Sideslip - ie banking into the operative engine will KILL YOU in an MU-2.
Half a ball deflection costs about 400fpm (or so I was told by an MU-2 veteran)

Varies with different types of airplane.

 

[MauleSkinner]

This is my experience on this subject.

Obviously, the rest of our experience disagrees.

David

 

[skiddriver]

Last post on this

Last post on this subject.

First, I have to ask, does anyone really think that there is no sideslip just because Dauntless' string didn't show one in his installation. The installation of aircraft test instrumentation is a science unto itself. Sideslip measuring vanes are not installed on the top of the nose of an aircraft due to the localized airflow effects there; they are installed in free air at the end of a pitot-static test boom. Dauntless' experience notwithstanding, in the described flight condition, there is a sideslip, and it does have an effect on performance.

When I said that the effect was small, when I review my data card from the flight, it had a 200 to 300 FPM positive effect, as measured on a sensitive (instrumentation grade) IVSI. This is one-time test day data in a specific aircraft. A "real" test program would have referred this data to test conditions and would have been conducted across a spectrum of conditions to achieve generalized data. If you are already at a significant positive rate of climb, 200 to 300 FPM added is probably not significant. If you need the climb rate, it may be. In any case, I believe, and have both the theoretical basis (as in having computed the partial differentials to prove the concept) as well as test data (taken in an instrumented aircraft) that show that there is a positive effect in the blended solution of arresting rotation with the rudders, and banking into the good engine approximately 3 degrees. This effect is going to vary between aircraft and given conditions of the day.

Dauntless, you'll probably think I'm being facetious, but I'm not. If you are interested in being a test pilot, contact the National TPS. You can buy yourself a seat there and will actually learn what it takes to conduct a valid test program. http://www.ntps.edu/

 

[A Squared]

When I said that the effect was small, when I review my data card from the flight, it had a 200 to 300 FPM positive effect, as measured on a sensitive (instrumentation grade) IVSI.

Hmmmm, I'm thinking that 200 fpm might be just what I need to make the difference between going *down* 100 fpm, and going *up* 100 fpm. Me, I'll take 100 fpm up over 100 fpm down any day. Undaunted can continue arguing the merits of 100 fpm down if he wants, it will be a tough sell.

 

[UndauntedFlyer]

Skiddriver: You are providing some good information about certain high performance military aircraft but have you done any tests on GA aircraft such as this thread discusses? I mean planes like the Seminole, the C310, the Baron or a C421?

I have never said that your tests are wrong, only than the performance enhancements and so small on GA aircraft that it isn't worth getting all wrapped up in thinking this will help a lot because I know it doesn't.

Also, do you have any idea why all this zero side slip is never mentioned in the FM for large 2-engine or 4-engine airline equipment? No one says nor does the FM say anything about this technique for engine failure at V1 at max GW when performance is very critical.

 

[A Squared]

Skiddriver: You are providing some good information about certain high performance military aircraft but have you done any tests on GA aircraft such as this thread discusses? I mean planes like the Seminole, the C310, the Baron or a C421?

Uhhhhhh, the U-21 of which he speaks is a king air 90. Not knocking the king air, but it's hardly a "high performance military aircraft"

So the answer woud be, yes, he has done tests in GA aircraft.

 

[minitour]

Hmmmm, I'm thinking that 200 fpm might be just what I need to make the difference between going *down* 100 fpm, and going *up* 100 fpm. Me, I'll take 100 fpm up over 100 fpm down any day. Undaunted can continue arguing the merits of 100 fpm down if he wants, it will be a tough sell.

Yep...100fpm in a light twin on one engine can be very exciting considering the alternatives.

-mini

 

[Skygod]

On the airbus, lose an engine on takeoff and the slip-skip indicator changes to become a beta target, and when centered with the rudder this results in best climb performance.

Don't know about Boeing, but seems Airbus trusts test pilots and their 'placebo.'